1. Field of the Invention
The present invention relates to a method and an apparatus for inspecting defects. More particularly, the present invention relates to a method and an apparatus for conducting a rapid and accurate inspection of a defect, for example, a particle and/or a scratch that exists on a semiconductor substrate.
2. Description of the Related Arts
To process massive data in a short time, a semiconductor device has been highly integrated. To manufacture the highly-integrated semiconductor device, a method of precisely depositing a thin film pattern on a semiconductor substrate is very important. Accordingly, an inspection process for determining a failure of the thin film pattern on the substrate is required. For example, after a patterning process for forming a minute pattern is performed, a defect such as a particle or a minute scratch may be generated thereon. Also, after a chemical mechanical polishing (CMP) process is performed on the substrate, the defect may be generated.
A conventional inspection apparatus only detects the presence of the defect on an object. The substrate having the defect is re-inspected by an inspector. The inspector inspects the substrate using a review tool and the inspector's naked eye.
As the semiconductor device is highly integrated, however, the defects uncovered by inspection on a single substrate have remarkably increased. Where tens of inspection defects have been uncovered in the past on the single substrate, hundreds or thousands of defects are found by inspection on a single substrate today.
Although the numbers of defects have remarkably increased as mentioned-above, the inspection apparatus has been slowly developed so that inspection time has also greatly increased. This decreases the productivity of the semiconductor devices.
A plurality of minute structures can be formed on a substrate. When these minute structures are inspected using available technology, the productivity of the semiconductor device decreases.
To prevent this decrease of the productivity, only selected substrates are inspected. Thus, an arbitrary substrate is selected from the entire production of substrates for inspection purposes. The selected substrate is only inspected to determine failures of the entire substrate. However, even though the inspection time is curtailed, the reliability of the inspection process is reduced contrarily.
Referring to FIG. 1, in step S11, an arbitrarily selected wafer is loaded into an inspection apparatus. In step S12, a light is irradiated onto the wafer. In step S13, the irradiated light is reflected from the wafer. In step S14, the reflected light is collected using a photomultiplier tube. In step S15, the photomultiplier tube is employed to calculate an optimal amplification ratio in accordance with the intensity of the collected light. In step S16, the photomultiplier tube amplifies the collected light in accordance with the amplification ratio. In step S17, the amplified light is converted into a digital signal. In step S18, the digital signal is stored in a server. In step S19, the digital signal is compared to stored predetermined information for a reference wafer to determine the presence of a defect on the selected wafer.
Meanwhile, a plurality of minute structures, for example a line, a space, a contact hole or a pattern, is formed on a wafer. The minute structures may be divided into a cell region, a peripheral region and a sense amplifier.
Generally, these regions are repeatedly arrayed on the wafer. The minute structures formed in a same region have a substantially equal reflectivity. However, the minute structures formed in different regions have a different reflectivity. Thus, when a substantially the same light is irradiated on the different regions, the reflected lights may have a different reflectivity.
Further, when the reflected lights having the different reflectivities are amplified by the same amplification ratio, image information in at least one region may result in failure.
In particular, the reflected lights are collected using the photomultiplier tube. The photomultiplier tube generates photoelectrons in accordance with intensities of the collected lights. The photoelectrons create photoelectric currents having different intensities in accordance with the number of the photoelectrons present. Since the irradiated lights have a relatively low intensity, amplifying the irradiated lights is required. Accordingly, the irradiated lights are amplified differently in accordance with the voltage of the photomultiplier tube. As a result, a high photoelectric current is obtained from the irradiated light having a low intensity by controlling the voltage of the photomultiplier tube.
The photoelectric current is converted into a digital signal. The digital signal includes the image information of the minute structure on which the light is irradiated. Thus, the shape of the minute structure may be determined by analyzing the digital signal. When the voltage of the photomultiplier tube is constant, the collected light is amplified by a substantially equal amplification ratio. However, since the minute structures have different reflectivity levels, the collected lights are converted into the digital signal having clear image information from amplifying the collected light by different amplification ratios. Namely, when the voltage of the photomultiplier tube is not desirably controlled, the minute structure having a high reflectivity is shown in relatively high bright image, while the minute structure having a low reflectivity is shown in relatively low dark image. As a result, the shape of the minute structures may not be accurately determined.
As described above, the minute structures positioned in a same region have an equal or a similar reflectivity. When the light scans from one region to another region on the wafer, the amplification ratio of the photomultiplier tube typically varies.
When the number of the defects on the wafer exceeds a predetermined number after primarily inspecting the wafer using the photomultiplier tube, equipment for manufacturing a semiconductor device is suspended. The wafer having excessive defects is reviewed using a review tool. Here, the shapes of the defects are verified by the inspector's naked eye using the review tool.
Accordingly, performing the primary inspection accurately using the photomultiplier tube is required. Since reviewing the wafer using the inspector's naked eye in a secondary inspection is restricted by the quality of sight of the inspector, error in the primary inspection needs to be reduced as much as possible. Further, when the defects are not precisely inspected in the primary inspection, the wafer having the defects may be transferred to a subsequent process line.
As mentioned above, reviewing the wafer using the inspector's naked eye is determined in accordance with result of the primary inspection using the photomultiplier tube. Entire processes are then determined based on this reviewing result. When the review is carried out based on inaccurate result of the primary inspection, unnecessary time loss may be induced and also the defects may not be accurately inspected.
Referring to FIG. 2, a conventional apparatus for inspecting a defect includes a light source 10 for irradiating a light onto a wafer W, a polarizer 20 for polarizing the irradiated light, a photomultiplier tube 30 for collecting light reflected from the wafer W, a power source 40 for providing current to the photomultiplier tube 30, a controller 50 for controlling a predetermined amplification ratio of the photomultiplier tube 30, and a central processor 60 for determining defects on the wafer W using the reflected light.
The wafer is disposed on a stage 70. The stage 70 is provided at a center of the inspection apparatus. The light source 10 is inclined at an angle of about 45° from the stage 70. The polarizer 20 is disposed between the light source 10 and the stage 70. The photomultiplier tube 30 is disposed at a position to readily collect the reflected light. An analog/digital (A/D) converter (not shown) is provided in the central processor 60. The A/D converter converts the reflected light into a digital signal stored in a server (not shown).
An operation algorithm of the photomultiplier tube 30 may be classified into a fixed gain type algorithm and an adaptive gain type algorithm. The fixed gain type algorithm is operated in accordance with a constant amplification ratio regardless of an amount and an intensity of the irradiated light. Since the amplification ratio of the photomultiplier tube 30 does not vary at the boundary at which the intensity of the irradiated light is changed, the image of an object such as the wafer W is light saturated.
The saturation of the image is illustrated in detail. The entire light reflected from the wafer W is amplified by substantially the same amplification ratio regardless of the intensity of the reflectivity varied in the fixed gain type algorithm. When output signals of the light reflected from one region of the wafer W having a high reflectivity, and from another region of the wafer W having a low reflectivity, are not amplified with different amplification ratios, the output signal of the light reflected from the region having the high reflectivity is higher than that of the light reflected from the region having the low reflectivity. Accordingly, the minute structure on the region having the low reflectivity is shown as a dim image.
The adaptive gain type algorithm is operated in accordance with different amplification ratios that vary according to an amount and an intensity of the irradiated light. That is, the amplification ratio of the light reflected from the high reflective region decreases, whereas that of the light reflected from the low reflective region increases. Thus, a dark region and a bright region are clearly distinguished in the adaptive gain type algorithm. As a result, the adaptive gain type algorithm has excellent inspection performance compared to that of the fixed gain type algorithm.
However, since a delay time occurs in the adaptive gain type algorithm, the saturation of the image is partially shown in the adaptive gain type algorithm. In particular, when the irradiated light is moved from the low reflective region, such as the cell region, to the high reflective region, such as the peripheral region or the sense amplifier, the delay time for sensing the moving light and for determining an optimal amplification ratio by the controller is required. The saturation of the image occurs using the adaptive gain type algorithm due to the delay time. As a result, the inspection performance of the conventional apparatus may be reduced.
Therefore, the saturation of the image deteriorates the performance of the inspection apparatus. When the defects are determined to be abnormal in accordance with information collected with the photomultiplier tube 30 in the delay time, the unnecessary reviewing of the wafer may be carried out. Further, since reviewing the wafer is performed through the inspector's manual operation, the time for reviewing the wafer is substantially longer than that required in using the automatic inspection apparatus.